No need to ask for permission if you look in the downloadable section as

Selected, pre-cleared Crown copyright images have been made available for use under the Open Government Licence (OGL). You are encouraged to use these images freely and flexibly, with only a few conditions. These images can be downloaded at high resolution from the "Downloadable" section of this website.

The images on this website that are available under the OGL are denoted with this logo next to them:

This file is available for reuse under the OGL (Open Government License).